Power steering pump leaking

I have a leak from my power steering pump.  Fluid leaks out of the top of this nut, just where the pipe goes into it.  When I take it off and replace the o-ring, it works fine for a few days then starts leaking again.

 

 

Does anyone have any idea what the problem could be, and how to get a more permanent fix?

 

Pull the pipe out and give it a close inspection for a pinhole leak. These pipes have been known to rust through